The prior art shows that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. See, for example, a commonly assigned U.S. patent application of R. A. Sprague et al., now U.S. Pat. No. 4,281,904 on a "TIR Electro-Optic Modulator With Individually Addressed Electrodes." Also see "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Wavforms," Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57.
As taught in the prior art, almost any optically transparent electro-optical material can be used as the electro-optic element of such a light valve. Some promising materials appear to be LiNbO.sub.3 and LiTaO.sub.3, but there are other materials which qualify for consideration, including BSN, KDP, KD.sup.x P, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT. In any event, the electrodes of such a light valve are intimately coupled to the electro-optic element and distributed in non-overlapping relationship widthwise of the electro-optic element (i.e. orthogonally relative to its opticl axis), typically on equidistantly separated centers so that there is a generally uniform inter-electrode gap spacing.
To perform line printing with such a multi-gate light valve, a photosensitive recording medium, such as a xerographic photoreceptor, is exposed in an image configuration as it advances in a cross line direction (i.e., a line pitch direction) relative to the light valve. More particularly, to carry out the exposure process, a sheet-like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Furthermore, successive sets of digital bits or analog signal samples (hereinafter collectively referred to as "data samples"), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized electric bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, in turn, cause localized variations in the refractive index of the electro-optic element within an interaction region (i.e., a light beam illuminated region of the electro-optic element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated (hereinafter generically referred to as "p-modulation" of the light beam) in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region. Schlieren readout optics may be used to convert a phase front modulated light beam into a light beam having a correspondingly modulated intensity profile. For example, the phase front modulated light beam may be imaged onto the recorded medium by central dark field or central bright field imaging optics. Alternatively, if the input light beam is polarized, the polarization modulation to intensity modulation conversion process may be performed by passing the polarization modulated output beam through a polarization analyzer. In more generic terms, the p-modulation of the light beam is converted into a correspondingly modulated intensity profile by using "p-sensitive readout optics" to image or project (hereinafter collectively referred to as imaging) the light beam onto the recording medium.
Unfortunately these prior art devices suffer from electrode aligned blind spots. Such blind spots exist because of the inherent electroe aligned nulls in such fringe fields. One prior art solution was taught by Robert A. Sprague in "Electro-Optic Line Printer", U.S. Pat. No. 4,389,659, issued June 21, 1983, and commonly assigned as the present application. The blind spot problem was avoided in this prior art patent by using a light valve which has a pair of longitudinally displaced, laterally staggered electrode arrays to produce essentially identical, laterally staggered fringe field patterns.
According to the present invention, similar longitudinally displaced, laterally staggered electrode arrays are used to produce the laterally staggered fringe field patterns. Since the impinging light beam will defocus while being deflected, means are provided to focus the beam into first and second interaction regions being addressed. A correcting lens can be provided of which position or its optical power can be dynamically controlled. A voice-coil activated lens can be utilized to control the variable position focussing lens.